Apparatus and methods for improving the intensity profile of a beam image used to process a substrate

ABSTRACT

Methods and apparatuses are provided for improving the intensity profile of a beam image used to process a semiconductor substrate. At least one photonic beam may be generated and manipulated to form an image having an intensity profile with an extended uniform region useful for thermally processing the surface of the substrate. The image may be scanned across the surface to heat at least a portion of the substrate surface to achieve a desired temperature within a predetermined dwell time. Such processing may achieve a high efficiency due to the large proportion of energy contained in the uniform portion of the beam.

BACKGROUND OF THE INVENTION

1. Field of Invention

The present invention relates to semiconductor processing methods andapparatuses that use one or more photonic beams having an initiallynonuniform intensity profile to generate an image, which, in turn, isscanned across a surface of a semiconductor substrate. In particular,the invention relates to such methods and apparatuses in which the imageexhibits a uniform intensity profile over a useful portion thereof suchthat energy utilization of the beam is increased.

2. Description of Background Art

Fabrication of semiconductor-based microelectronic devices such asprocessors, memories and other integrated circuits (ICs) often involvessubjecting a semiconductor substrate to numerous processes, such asphotoresist coating, photolithographic exposure, photoresistdevelopment, etching, polishing, and heating or “thermal processing”. Incertain applications, thermal processing is performed to activate dopantatoms implanted in junction regions (e.g., source and drain regions) ofthe substrate. For example, the source/drain parts of transistors may beformed by exposing regions of a silicon wafer to electrostaticallyaccelerated dopants containing either boron, phosphorous or arsenicatoms. After implantation, the dopants are largely interstitial, do notform part of the silicon crystal lattice, and are electrically inactive.Activation of these dopants may be achieved by annealing the substrate.

Annealing may involve heating the entire substrate to a particularprocessing temperature for a period of time sufficient for the crystallattice to incorporate the impurity atoms in its structure. The requiredtime period depends on the processing temperature. Particularly duringan extended time period, the dopants tend to diffuse throughout thelattice. As a result, the dopant distribution profile may change from anideal box shape to a profile having a shallow exponential fall-off.

By employing higher annealing temperatures and shorter annealing timesit is possible to reduce dopant diffusion and to retain the dopantdistribution profile achieved after implant. For example, thermalprocessing (TP) encompasses certain techniques for annealingsource/drain regions formed in silicon wafers as part of the process forfabricating semiconductor devices such as integrated circuits (ICs). Anobjective of rapid thermal processing (RTP) is to produce shallow dopedregions with very high conductivity by rapidly heating the wafer totemperatures near the semiconductor melting point to Incorporate dopantsat substitutional lattice sites, and then rapidly cool the wafer to“freeze” the dopants in place. RTP is particularly useful in the contextof semiconductor-based microelectronic devices with decreased featuresizes, because it tends to produce low-resistivity doped regions, whichtranslates into faster ICs. It also results in an abrupt change indopant atom concentration with depth as defined by the implant process,since thermal diffusion plays only a very minor role in therearrangement of the impurity atoms in the lattice structure.

Laser-based technologies have been employed to carry out TP on timescales much shorter than those employed by conventional RTP systems.Exemplary terminology used to describe laser based TP techniques includelaser thermal processing (LTP), laser thermal annealing (LTA), and laserspike annealing (LSA). In some instances, these terms can be usedinterchangeably. In any case, these techniques typically involve forminga laser beam into a long, thin image, which in turn is scanned across asurface to be heated, e.g., an upper surface of a semiconductor wafer.For example, a 0.1-mm wide beam may be raster scanned over asemiconductor wafer surface at 100 mm/s to produce a 1-millisecond dwelltime for the heating cycle. A typical maximum temperature during thisheating cycle might be 1350° C. Within the dwell time needed to bringthe wafer surface up to the maximum temperature, a layer only about 100to about 200 micrometers below the surface region is heated.Consequently, the bulk of the millimeter thick wafer serves to cool thesurface almost as quickly as it was heated once the laser beam is past.Additional information regarding laser-based processing apparatuses andmethods can be found in U.S. Pat. No. 6,747,245 and U.S. PatentApplication Publication Nos. 2004/0188396, 2004/0173585, 2005/0067384,and 2005/0103998 each to Talwar et al.

LTP may employ either pulsed or continuous radiation. For example,conventional LTP may use a continuous, high-power, CO₂ laser beam, whichis raster scanned over the wafer surface such that all regions of thesurface are exposed to at least one pass of the heating beam. Thewavelength of the CO₂ laser, λ, is 10.6 μm in the infrared region. Thiswavelength, large relative to the typical dimensions of wafer features,can be uniformly absorbed as the beam scans across a patterned siliconwafer resulting in each point on the wafer being subject to very nearlythe same maximum temperature.

Similarly, a continuous radiation source in the form of laser diodes maybe used in combination with a continuous scanning system. Such laserdiodes are described in U.S. Pat. No. 6,531,681, entitled “ApparatusHaving Line Source of Radiant Energy for Exposing a Substrate”, whichissued on Mar. 11, 2003 and is assigned to the same assignee as thisapplication. Laser diode bar arrays can be obtained with output powersin the 100 W/cm range and can be imaged to produce line images about amicrometer wide. They are also very efficient at converting electricityinto radiation. Further, because there are many diodes in a bar eachoperating at a slightly different wavelength, they can be imaged to forma uniform line image.

An alternate method of annealing employs a pulsed laser to illuminate anextended area and a step-and-repeat system. In this case, a more uniformtemperature distribution can be obtained with a longer radiation pulse(dwell time) since the depth of heating is greater and there is moretime available during the pulse interval for lateral heat conduction toequalize temperatures across the circuit. However, longer dwell timesrequire more pulse energy. Pulse lengths with periods longer than amicrosecond and covering circuit areas of 5 cm² or more are nottypically feasible because the energy per pulse becomes too high. Whiletechnically possible, the laser and associated power supply needed toprovide such a high-energy pulse will likely be impractically big andexpensive.

In general, illumination uniformity (both macro- and micro-uniformity)over the useable portion of the exposure image is a highly desirabletrait. This ensures that the corresponding heating of the substrate isequally uniform. Similarly, the energy delivered in each beam pulseshould be stable so that all exposed regions are successively heated toa uniform temperature. In such a system, the size of the uniformlyilluminated area may be adjusted to contain an integer number ofcircuits. In addition, the illumination fall-off beyond the edge of theusable portion of the exposure image is preferably sufficiently sharp,so that there is no appreciable exposure of adjacent circuits on thesubstrate. Defining the edges of the illumination pattern with aresolution of about 50 microns is usually sufficient since the scribelines separating adjacent circuits are typically at least that wide.

Certain “nonmelt” LTP techniques involve shaping the beam from acontinuous CO₂ laser to form an image of about 0.12 mm wide and over 10mm long, which is incident on the wafer at Brewster's angle (˜75°incidence). It is desirable to have the beam incidence angle containedin a plane normal to the wafer surface and aligned with the length ofthe image. The beam is scanned over the substrate in a directionperpendicular to its long direction. Even if a beam intensity uniformityof 1% can be achieved over the length of the image, this results in acorresponding 10° C. or 14° C. temperature difference along the beamdepending on whether the background substrate temperature starts at 400°C. or room temperature, respectively.

In some instances, then, e.g., for semiconductor annealing applications,a highly uniform intensity along the length of the beam, e.g., to about1%, may be desired. In this case if a beam having a Gaussian intensityprofile is employed, only the central portion of the beam that exhibitsa substantially uniform intensity, e.g., to about 1% or less, may beused. This useful portion contains only about 11% of the total energy inthe beam. The remaining energy may be wasted or may contribute toundesirable heating of adjacent regions.

Thus, opportunities exist in the art to improve the performance of TPtechniques to overcome the drawbacks associated with known LTPtechniques that involve the use of one or more radiation beams having anonuniform intensity profile. In addition, there exist opportunities inthe art to meet the need for LTP technologies that exhibit improvedenergy utilization.

SUMMARY OF THE INVENTION

The invention provides methods and apparatuses for processing asemiconductor substrate having a surface with radiation. In oneembodiment, the inventive method involves generating at least onephotonic beam having a nonuniform intensity profile and using the atleast one photonic beam to form an image on the substrate surface. Theimage may exhibit a uniform intensity profile over a useful portionthereof and the proportion of energy in the useful portion relative tothe rest of the image may be a measure of the energy utilization. Inaddition, the image may be scanned across the substrate surface toachieve a desired temperature within a predetermined dwell time, D.

In some instances, the at least one photonic beam is manipulated torender at least a portion of its intensity profile more uniform. Forexample, the intensity profile of the beam may initially besubstantially Gaussian, possibly with some contributions from higherorder modes. After manipulation, the beam may form a line image havingan intensity profile along its length more boxcar in shape. Beammanipulation is particularly suited for methods that start with a singlephotonic beam having a nonuniform intensity profile.

Two or more photonic beams may be used to form a single contiguousimage. In such a case, some or all input beams may have a nonuniformintensity profile and the combination a uniform intensity profile over auseful portion thereof that is longer than provided by any of the inputbeams.

When a plurality of beams is used, it is often, but not always,preferable that the beams be combined in a substantially noninterferingmanner to form a contiguous image. A number of techniques may be used toavoid the adverse consequences of substantial beam interference. Forexample, interference between two beams forming a contiguous image maybe effectively avoided if their frequencies are varied so that thetransitory period when the frequencies are sufficiently close to createsignificant interference effects is a small fraction of the dwell time.In this case the net result on the peak temperature, which depends onthe intensity integrated over the whole dwell time period, is likely tobe negligible. In addition or in the alternative, each beam may have awavelength that is locked relative to another with a frequencydifference separating the beams that is much greater than 1/D.

Regardless of the number of beams used to create the image on thesubstrate, the useful portion of the image is the portion in closeproximity to the peak intensity having a substantially uniform intensityprofile. The useful portion typically contains a peak intensity and theintensity profile of the useful portion may be entirely within a rangeof about 98% to 100% of the peak intensity. Optionally, the intensityprofile may be entirely within about 99% to 100% of the peak intensity.

The invention is typically used to effect rapid semiconductor annealing.Accordingly, the desired temperature is typically sufficient toelectrically activate dopant atoms implanted or otherwise placed into asemiconductor material. For silicon, the desired temperature may be atleast about 1300° C. and lower than silicon's melting point.

For rapid annealing, either pulsed or continuous beams may be usedhaving a predetermined dwell time of no more than about 10 milliseconds.Such short dwell times may involve using a beam having an average powerof at least 250 W, 1000 W, 3,500 W, or more depending on the dwell timeand the image size. Higher beam energy utilization is preferred for anumber of reasons, e.g., to reduce the laser power requirements and toincrease the throughput of the tool. Accordingly, it is desirable toachieve an energy utilization of at least 15%, 25%, 35%, and preferablymore. Optionally, one or more beams may be incident to the substratesurface at or near the surface's Brewster's angle.

In another embodiment, an apparatus is provided for processing asubstrate having a surface. The apparatus may include a radiation sourceadapted to emit a photonic beam of a nonuniform intensity profile and astage adapted to support a substrate having a surface. An optical systemmay be adapted to receive the emitted beam and create therefrom an imageon the substrate surface. A controller may be operably coupled to theradiation source and the stage. The image may exhibit a uniformintensity profile over a useful portion thereof such that energyutilization of the beam is high. The controller is programmed to providerelative movement between the stage and the beam to scan the imageacross the substrate surface to heat at least a portion of the substratesurface to achieve a desired temperature within a predetermined dwelltime, D.

Optionally, a plurality of radiation sources may be used, and theoptical system may be adapted to receive and combine beams from thesources to form a contiguous image on the substrate surface. A number ofmeans may be used to ensure that the beams do not substantiallyinterfere with each other. For example, the cavity length of each lasermay be modulated at a different frequency so the period of time that anytwo laser frequencies correspond is very short compared to the dwelltime. This condition may be satisfied when the cavity mirror modulationfrequency difference from laser-to-laser (Δf) is greater than 1/D andthe modulation amplitude is sufficient to produce a shift in laserfrequency corresponding to or greater than 9000(Δf). In order tosuppress diffraction from the apertures used to combine the beams it isnecessary to combine the beams near their waist positions and provide abeam separation distance of at least six times the 1/e² waist intensityradius between beams. In addition or in the alternative, when aradiation source is a laser, the laser may have a variable cavitylength. In such a case, the controller may be programmed to providevariations in cavity length in a manner effective to produce alaser-to-laser wavelength change frequency that substantially reduces oreliminates adverse interference effects. Laser diode technology may beused as well to provide all of the energy for annealing or to supplementthe beam from a single large laser.

Regardless of how many radiation sources are used, variations on theoptical system may be used. For example, the optical system may includea reflective aspheric element having a surface adapted to transform thenon-uniform intensity profile of a photonic beam emitted from theradiation source into a uniform image on the substrate surface. Theaspheric surface may be fixed or adjustable. Means may be provided foraltering beam size and position on the reflective aspheric and theaspheric element may have an adjustable surface profile.

In addition or in the alternative, the optical system may include arefractive element. Furthermore, the optical system may include an arrayof waveguides. Waveguides may be arranged to alter the nonuniformintensity profile of photonic beam emitted from the radiation source tocreate the image on the substrate surface.

In any case, the optical system is typically configured to create fromthe emitted beam a line image having a lengthwise axis on the substratesurface. In addition, the stage is typically adapted to hold thesubstrate in a position to receive radiation from the optical system atan incident angle greater than 45°. The beam forming optical system maybe arranged so that the incident angle is contained in a plane normal tothe substrate surface and containing the lengthwise axis of the image.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates an exemplary single-laser thermalprocess apparatus for effecting rapid thermal processing of a substratethat may be used with the invention.

FIG. 2 is a plot that models how an input beam with a Gaussian intensityprofile may be converted using an optical element having a correctionfunction into an output beam with a substantially uniform intensity.

FIG. 3 is a plot that models how shifts in relative positions of theinput beam and the optical element of the model associated with FIG. 2may affect the intensity profile of the output beam.

FIG. 4 is a plot that models how beam diameter changes (beam breathing)associated with the input beam of the model associated with FIG. 2 mayaffect the intensity profile of the output beam.

FIG. 5 schematically illustrates the geometries associated with anexemplary ray-tracing model of a reflective aspheric meant to alter thebeam intensity distribution of the wafer.

FIG. 6 is a plot generated from of ray-tradng based model thatapproximates an ideal aspheric surface that may be used with theinvention.

FIG. 7 is a plot that shows an idealized relationship between the rayposition on the substrate versus its position on the aspheric surface ofthe model associated with FIG. 6

FIG. 8 is a plot that shows the intensity profile of an image formed onthe substrate from a Gaussian beam reflected from the aspheric elementof the model associated with FIG. 6 with and without some edgemodifications.

FIG. 9 is a plot that shows the effects of beam decentering on theIntensity uniformity of the output beam of the model associated withFIG. 6.

FIG. 10 is a plot that shows the effects of beam size variations on theintensity uniformity of the output beam.

FIG. 11 is a normalized plot of the intensity profiles of three inputbeams having different intensity profiles.

FIG. 12 is a plot of the intensity profiles of the output beams formedfrom three input beams of FIG. 11 as a result of reflection from anaspheric surface.

FIG. 13 is a plot of the intensity profiles of the output beams formedfrom three beams with differing profiles after adjustments to theirwidth and after reflection from an aspheric surface.

FIG. 14 is a plot of two aspheric surface profiles, one unmodified andthe other unmodified.

FIG. 15 is a plot of the intensity profiles of output beams formed fromthree adjusted input beams as a result of reflection from a modifiedaspheric surface.

FIG. 16 schematically illustrates an optical element in the form of apower adjusting mirror that may be used to change a beam's diameter.

FIG. 17 schematically illustrates an optical element in the form of anadjustable aspheric mirror.

FIG. 18 schematically illustrates another optical element having anadjustably deformable surface.

FIG. 19 schematically shows an exemplary optical system for combiningtwo laser beams in accordance with the invention.

FIG. 20 is a plot of the intensity profiles of two individual Gaussianbeams and their combined intensity profile.

The drawings are intended to illustrate various aspects of theinvention, which can be understood and appropriately carried out bythose of ordinary skill in the art. The drawings may not be to scale ascertain features of the drawings may be exaggerated for emphasis and/orclarity of presentation.

DETAILED DESCRIPTION OF THE INVENTION Definitions and Overview

Before describing the present invention in detail, it is to beunderstood that this invention, unless otherwise noted, is not limitedto specific substrates, temperature measuring means, or materials, allof which may vary. It is also to be understood that the terminology usedherein is for the purpose of describing particular embodiments only, andis not intended to be limiting.

It must be noted that, as used in this specification and the appendedclaims, the singular forms “a”, “an” and “the” include both singular andplural referents unless the context clearly dictates otherwise. Thus,for example, reference to “a beam” includes a plurality of beams as wellas a single beam, reference to “a wavelength” includes a range orplurality of wavelengths as well as a single wavelength, reference to “aregion” includes a combination of regions as well as single region, andthe like.

In describing and claiming the present invention, the followingterminology will be used in accordance with the following definitions.

The term “apodization” is generally used to describe the manipulation ofthe amplitude and/or phase of a photonic beam, typically for the purposeof producing a more desirable image. As a related matter, the term“aspheric” is used to describe an optical element having a surface thatdeparts from being flat, spherical or cylindrical and which may serve tocorrect undesirable optical aberrations or to produce desired intensityprofiles in beams or images. For example, an “aspheric surface” may havea surface profile that generally follows the shape of a “w”, and thatmay serve to render the intensity profile of an image more uniform.

The terms “Brewster's angle” or “Brewster angle” is used to refer to theangle of incidence between a radiation beam and a dielectric surfacethat corresponds to the minimum or near-minimum reflectivity of theP-polarized component of the beam. Films on the surface of an object,such as a silicon wafer, may prevent it from exhibiting zeroreflectivity at any angle. If, however, the films are dielectric innature, then there generally will be an angle of minimum reflecivity forP-polarized radiation. Accordingly, the Brewses angle as used herein fora specular surface formed from a variety of different dielectric filmsstacked on a substrate can be thought of as an effective Brewster'sangle, or the angle at which the reflectivity of P-polarized radiationis at a minimum. This minimum angle typically coincides with or is nearthe angle of the Brewster's angle for the substrate material.

The term “energy utilization” as in the “energy utilization of an image”refers to the proportion of energy associated with the portion of theimage useful for producing a desired effect relative to the total beamenergy in the image. For example, in an annealing application the“useful portion” of an image may be only that part of the beam thatcomes within about a percent or two of the maximum or peak beamintensity. A small modification to the image profile shape can produce alarge change in the “energy utilization”.

The term “intensity profile” in reference to an image or a beam refersto the distribution of radiation intensity along one or more dimensions.For example, an image may have a useful portion and a nonuseful portion.The useful portion of an image typically has a “uniform” intensityprofile that exhibits substantially the same intensity. In other word,the intensity profile integrated in the scan direction throughout theuseful portion of the image may be substantially constant. Accordingly,any point on a substrate surface region that is scanned by a usefulportion of an image having a uniform intensity profile will be heated tothe same temperature. However, the intensity or intensity profile of thenonuseful portion may differ from that of the useful portion. Thus, theimage as a whole may have an overall “nonuniform” intensity profile eventhough a useful portion by itself may exhibit a uniform intensityprofile.

As a related matter, the term “peak intensity value” of an image or abeam refers to the point of highest intensity in the image or beam.Typically, the entirety of the useful portion of an image will exhibitthe peak intensity.

The term “light emitting photodiode (LED)” refers to a diode that istypically made from semiconductor material, and which converts anapplied voltage to photonic radiation. The terms “diode” and “LED” aregenerally interchangeably used herein, however this is not to beconfused with the term “photodiode”, which may refer to a photo-detectorthat converts incident radiation into an electrical current. The term“laser diode” refers to a type of LED that emits coherent light whenforward biased.

The term “semiconductor” is used to refer to any of various solidsubstances having electrical conductivity greater than insulators butless than good conductors, and that may be used as a base material forcomputer chips and other electronic devices. Semiconductors includeelements such as silicon and germanium and compounds such as siliconcarbide, aluminum phosphide, gallium arsenide, and indium antimonide.Unless otherwise noted, the term “semiconductor” includes any one or acombination of elemental and compound semiconductors, as well asstrained semiconductors, e.g., semiconductors under tension orcompression. Exemplary indirect bandgap semiconductors suitable for usewith the invention include Si, Ge, and SiC. Direct bandgapsemiconductors suitable for use with the invention include, for example,GaAs, GaN, and InP.

The terms “substantial” and “substantially” are used in their ordinarysense and refer to matters that are considerable in importance, value,degree, amount, extent or the like. For example, the phrase“substantially Gaussian in shape” refers to a shape that correspondspredominantly to the shape of a Gaussian probability distribution curve.However, a shape that is “substantially Gaussian” may exhibit somecharacteristics of a non-Gaussian curve as well, e.g., the curve mayalso include a component described by a polynomial. Similarly, a“substantially uniform” intensity profile will contain a relatively flatportion where the intensity does not deviate more than a few percentfrom the profile's peak intensity. Preferably, the intensity deviationis less than about 2%. Optimally, the intensity deviation is no morethan about 1% or no more than about 0.8%. Other uses of the term“substantially” involve an analogous definition.

The term “substrate” as used herein refers to any material having asurface, which is intended for processing. The substrate may beconstructed in any of a number of forms, for example, such as asemiconductor wafer containing an array of chips, etc.

In general, the invention involves processing methods and apparatusesthat use one or more photonic beams having, in some cases, a nonuniformintensity profile. While the invention may be practiced using one ormore photonic beams having an arbitrary intensity profile, the inventionis particularly suited for high-power photonic beams such as thosegenerated by a laser that exhibit a substantially Gaussian intensityprofile. The one or more high-power beams, e.g., having a power of atleast 250 W, 1000 W, or 3500 W, may be used to generate an image, which,in turn, is scanned across a surface of a substrate to effect rapidthermal processing, e.g., melt or nonmelt processing, of the substratesurface. Such powers may provide exposure energy doses of at least about0.1 or about 0.5 j/cm² to about 1 J/cm² over a dwell time. These methodsand apparatuses are particularly suited for annealing semiconductorwafers containing microelectronic devices such as processor and memorychips.

The invention represents an improvement over known technologies in thatit provides a more uniform processing image intensity profile. Inparticular, an image generated using the invention may exhibit a greateruseable portion than an image generated without using the invention. Asa result, energy utilization of the beam may be increased. For example,the invention may increase energy utilization by 30%, 100% or more. Suchincreased energy utilization tends to result in improved performancefrom a throughput perspective, whereas better uniformity may improveperformance from a quality perspective. Increased energy utiliation anduniformity may serve to improve overall performance in a synergisticmanner.

To provide context to the nature of the invention, it should be notedthat LTP typically involves rapidly heating a wafer surface totemperatures near the semiconductor melting point, e.g., to at leastabout 1300° C. in the case of silicon wafers, to incorporate dopants atsubstitutional lattice sites. Then, the wafer is rapidly cooled to“lock” the dopants in place where they are electrically active. Thisactivation process can be carried out, for example, by a pulsed laserand a step-and-repeat stage motion, or by a continuous laser and ascanning stage. The beam from the pulsed laser is generally shaped tocover an area encompassing a number of circuits, whereas the scannedbeam is usually shaped into a long, narrow beam under which thesubstrate can be scanned in a direction orthogonal to the longdirection. The dwell time may be determined by the duration of the laserpulse or the width of the laser beam divided by the scanning velocity.With sufficiently short dwell times, only a shallow region just belowthe surface is heated to high temperature, and the bulk of the waferserves to cool the region almost as quickly as it was heated.

When the beam image on the substrate has a nonuniform intensity profile,the surface temperature and the resultant thermal processing is alsononuniform. Over-heating the surface generally results in irreparabledamage so this is not a viable option for thermal processing. Thus, foran image with a Gaussian intensity profile, effectively only the centralportion of the image with the highest intensity can serve as a usefulportion for thermal processing. For example, if a 1% annealinguniformity is required then the proportion of energy associated with theuseful central portion is about 11% of the total energy in the Gaussianimage, and about 89% of the total beam energy, is effectively wasted. Inother words, a typical energy utilization of a beam image having aGaussian intensity profile used for thermal processing of asemiconductor wafer is less than about 11%.

The invention provides for an increased energy utilization of at least15%, 25%, 35%, or more. When calculated relative to the less than about11% energy utilization of a putative unmodified image formed by a beam,the actual image of the invention may exhibit an energy utilization thatis at least about 30% greater than the energy utilization of theputative unmodified image. For example the invention may be used toimprove energy utilization of known apparatus, e.g., by at least about30%, 100%, 200%, or more.

A number of advantages can be realized by improving intensity uniformityand/or increasing energy utilization. As an initial matter, the costassociated with low energy utilization may be reduced. In addition,improved beam utilization may result in increased thermal processingthroughput, whereas improved uniformity may result in a higher qualityproduct. Very often improved uniformity leads directly to improvedutilization as well.

The better beam uniformity and improved energy utilization aspects ofthe invention may be achieved in a number of ways. As discussed above,some embodiments of the invention involve a method in which at least onephotonic beam having a nonuniform intensity profile is used to form animage on the substrate surface, and the image is scanned across thesubstrate surface. A useful portion of the image has a uniform intensityprofile that allows it to heat at least a portion of the substratesurface to achieve a desired uniform temperature within a predetermineddwell time. If the dwell time of the beam on a fixed point on thesurface is short enough, then the depth of heating may not penetratevery far below the substrate surface. Depending on the particulars ofthe embodiment, reflective, refractive, diffractive, additive,subtractive, interfering, noninterfering, and/or other technologies formanipulating photonic beams may be used to produce an image with a moreuniform intensity profile.

FIG. 1 is a schematic diagram of a simplified exemplary embodiment of athermal processing apparatus 10 that may be used to anneal and/orotherwise thermally process one or more selected surface regions of asubstrate according to the present invention. LTP system 10 includes amovable substrate stage 20 having an upper surface 22 that supports asemiconductor substrate 30 having an upper surface P. Substrate stage 20is operably coupled to controller 50. Substrate stage 20 is adapted tomove in the X-Y plane (as well as along the Z-axis) in a transverseand/or rotational manner under the operation of controller 50 so thatthe substrate can be scanned relative to the image generated fromradiation provided by radiation source 110.

The radiation source 110 is operably coupled to controller 50, and anoptical system 120 that serves to produce from radiation generated bythe radiation source one or more beams that are in turn imaged on thesubstrate. In an exemplary embodiment, radiation source 110 is a CO₂laser that emits radiation at a wavelength λ_(H) ˜10.6 μm in the form ofa beam that has a substantially Gaussian intensity profile. However, theradiation source may employ LED or laser diode radiation as well, e.g.,radiation having a wavelength of about 0.8 μm. Optionally, a pluralityof radiation sources may be employed. As shown, the laser 110 generatesan input beam 112 that is received by an optical system 120 that isadapted to manipulate the input beam to produce an output beam thatforms an image on the substrate that exhibits an intensity profile, theuseful portion of which is uniform. As shown schematically, the opticalsystem 120 includes a first reflector 122 and a second reflector 124. Inpractice any number of optical elements may be used some of which may bein the form of a reflector such as an aspheric mirror and/or acylindrical mirror. Piezoelectric actuators on reflectors such as thefold mirrors can be used to stabilize the position and the pointingangle of the beam. Similarly mirrors with adjustable power can be usedto change the beam size and adjustable aspheric mirrors can serve toimprove the image intensity profile produced by the optical system.

The optical system may vary according to the requirements of theapparatus. For example, any one or a combination of reflective,refractive, and diffractive elements may be used. In addition or in thealternative, the input beam may be transformed into the output beamusing an optical system that includes a plurality of waveguides oroptical fibers, each having a receiving terminus and an emittingterminus. The waveguides or fibers may be arranged such that receivingtermini thereof form a receiving array that intercepts some, most, orsubstantially all of the input beam and that emitting termini thereofform an emitting array having an amplitude and phase distributionrequired to form an image with an extended uniform section.

Through judicious mapping of the corresponding locations of thereceiving and emitting termini in the emitting array, an output beam canbe created that is relatively independent of the shape and position ofthe input beam. For example, the input and output ends of the waveguidesmay be arranged so that the image formed on the substrate surface has ahigher proportion of energy contained in the useful portion of the imageafter a displacement of the beam with respect to the input end of thearray than would be the case without the waveguide array. In addition,the ends may be arranged so that the image formed on the substratesurface has a higher proportion of energy contained in the usefulportion of the image after a change in the size of the beam at the inputend of the array than would be the case without the waveguide array.Also by careful adjustment of the phase and amplitude of the outputbeam, it is possible to create an image with the desiredcharacteristics; e.g., a uniform intensity profile. Variationspertaining to optical systems that may be used with the invention arediscussed in greater detail below.

In any case, the optical system 120 transforms the input beam 112 intooutput beam 140. Beam 140 travels along optical axis A, which makes anangle θ with a substrate surface normal N_(S). Typically, it is notdesirable to image a laser beam on a substrate at normal incidence,because any reflected light may cause instabilities when it returns tothe laser cavity. Another reason for providing optical axis A at anincident angle θ other than at normal incidence, is that efficientlycoupling of beam 140 into the substrate 30 may best be accomplished byjudicious choice of incident angle and polarization direction, e.g.,making the incident angle equal to the Brewster's angle for thesubstrate and using p-polarized radiation. In any case, the stage may beadapted to scan the substrate through the beam position while preservingthe incident angle.

Beam 140 forms image 150 at substrate surface P. In an exemplaryembodiment, image 150 is an elongate image, such as a line image, havingits lengthwise boundaries indicated at 152, and located within a planecontaining the incident beam axis and the surface normal. Accordingly,the incident angle of the beam (θ) relative to the substrate surface maybe measured in this plane.

The controller may be programmed to provide relative movement betweenthe stage and the beam. As a result, the image may be scanned across thesubstrate surface to heat at least a portion of the substrate surface.Such scanning may be carried out in a manner effective to achieve adesired temperature within a predetermined dwell time, D. Scanning maytypically be performed in a direction that is orthogonal to thelengthwise axis of the image although this is not a requirement.Nonorthogonal and nonparallel scanning may be carried out as well. Ameans may also be included to provide feedback as to the effectivenessof the intensity uniformization technology. Various temperaturemeasuring means and methods may be used with the invention. For example,a detector array, with each detector optionally keyed to specificlocation on the substrate surface might be used to measure thetemperature distribution over the surface or the maximum temperature asa function of the position across the length of the beam image.Optionally, a means for measuring the intensity profile of the beam onthe substrate may be used as well.

Optimally, a real-time temperature measurement system may be employedthat can sense the maximum temperature with a spatial resolution smallerthan thermal diffusion distance and with a time constant less than orcomparable to the dwell time of the scanned beam. For example, atemperature measurement system may be used that samples the emittedradiation 20,000 times a second at 256 points spread evenly over a 20 mmline-image length. In some instances, 8, 16, 32, 64, 128, 256, 512, ormore distinct temperature measurements may be made at a rate of 100,1000, 10,000, 50,000 line scans per second. An exemplary temperaturemeasurement system is described in U.S. patent application Ser. No.11/129,971, entitled “Methods and Apparatus for Remote TemperatureMeasurement of a Specular Surface”, filed on May 16, 2005. Suchtemperature measurement systems may be used to provide input to thecontroller so that appropriate corrections can be made possibly byadjusting the radiation source and/or the optical system.

Another exemplary temperature measurement system employs a linear arrayof InGaAs detectors, each 50 micrometers wide and 500 micrometers high,which are employed in the wavelength band from 1.5 μm to 1.7 μm. Eachdetector is positioned to receive p-polarized radiation emited directlyfrom the substrate at Brewster's angle so the substrate closelyresembles a black body. Periodically, there is superimposed on the fieldof view of the linear detector array a radiation sample that has beencollected from the opposite azimuth angle and retro-reflected from theportion of the substrate under observation. If the substrate is truly ablack body, there is no reflection of this radiation and the signalremains the same. If the portion of the substrate under observation isnot a perfect black body then some of the retro-reflected radiation isreflected from the wafer surface and onto the detector array to increasethe signal level. The change in signal level allows the emissivity ofthe wafer to be calculated on a point-by-point basis and accounted forwhen converting the radiation measurement into a temperature estimate.

In any case, the invention may also provide modules that may beincorporated into or used to modify existing thermal processingequipment, laser-based or otherwise. The modules include, for example:temperature measurement means; active optical elements, such asdeformable mirrors for altering the beam intensity profile and/or shape;mirror actuators to correct beam pointing and position errors, and meansfor maintaining the beam size. In some instance, use of these modulesmay obviate the need of selected components of existing laser thermalprocessing equipment, e.g., magnification relays.

Apodization Technologies

Regardless of the apparatus particulars, one or more beams having anonuniform intensity profile may be manipulated so that when imaged onthe substrate the length over which the intensity profile has acceptableuniformity is increased. Generally, apodization techniques for renderinga uniform beam image involve precise and accurate control of the shape,size, position and direction of an input beam and very precise controlof the imaging optical system. It has been demonstrated that currentstate-of-the art diffraction optical elements may sometimes be used toachieve a flat, top-hat beam profile within 5%. The main limitation toachieving a profile better than 5% is mainly variations in the profileof the input beam and imperfections in the optical elements disposedbetween the radiation source and the substrate.

Aspheric Elements

In some instances, the intensity profile of the input beam may besubstantially Gaussian in shape. Optionally, the intensity profile mayinclude in addition some higher order Hermite-gaussian eigenmodes thatalter the beam shape. After passing through the imaging system, theinput beam is transformed into an output beam, which when imaged mayhave an intensity profile that is more box-car (in the context of lineimages) or top hat (in the context of round spot images) in shape. Thismay be done, for example, using various known techniques known in theart. In particular, it has been demonstrated that an input beam havingan intensity profile that is substantially Gaussian in shape may beconverted into an output beam image with an intensity profile that ismore boxcar in shape using a mirror having an aspheric surface. Ineffect, such an aspheric surface expands the useful portion of theoutput beam. Another advantage of adding the aspheric surface is thatthe aspheric can be employed to assist in generating a sharp cutoff atthe beam edge. As discussed below, design, construction andimplementation of aspheric elements may be based on results obtainedfrom various modeling techniques.

Generally, aspheric elements of the invention may involve a reflectiveor refractive surface that is “w” shaped in one cross-section and flatin the orthogonal direction. The total departure from a flat plane maybe only a few microns to a hundred microns depending on the position ofthe element in the optical train. In some instances, it may be difficultto fabricate an aspheric element having a reflective surface thatexhibits exactly the desired profile, and the desired profile may changeslightly with time. However, when the desired amount of asphericdeparture from the surface profile of a widely available optical elementis small, e.g., ±5 micrometers, and the element is reflective it issometimes possible to distort the element under appropriate forces so asto shape the surface such that it precisely and accurately conforms tothe desired profile, at least temporarily. Adjustable apodizationtechnologies such as deformable mirror elements are discussed below.

Beam Position and Size

Beam position and stability, particularly relative to the beam shapingoptical systems described in the invention, are typically importantfactors that affect the quality of the processing image. In the contextof thermal processing of semiconductor wafers, the beam position shouldbe maintained to a small fraction of a percent of the beam width.Furthermore, the initial beam intensity profile should be accuratelyknown and be maintained substantially constant as well.

For example, for laser beam position may vary over a two millimetersrange at a point eight meters from the laser because of pointinginstability. In addition, the nominal beam width may vary by about fiveparts in eighty. These changes may occur in a fraction of a second.There may also be temporal changes in the relative proportions of loworder modes making up the beam profile, that occur slowly over weeks oftime and contain as much as 5% of the energy in the beam.

When a Gaussian laser beam is not properly centered on the opticalsystem that uses an aspheric element, the beam intensity profilegenerated as a result may be suboptimal. The deleterious effects onimage generation may be approximated by assuming the aspheric elementoperates on the Gaussian profile in much the same way as the reciprocalGaussian function operates on a Gaussian function when they aremultiplied together. This is shown in FIG. 2. When both the Gaussiancurve and its inverse function are properly aligned and multipliedtogether the result is a flat boxcar function shown as the aligned curvein FIG. 3. As the Gaussian input beam shifts with respect to the beamshaper, a flat tilted curve results, and the degree of tilt varies withthe amount of decenter in a roughly linear fashion. In this example, atotal nonuniformity of 1% is produced by a decenter of about 1/164 ofthe beam width measured at the 50% points.

Thus, for thermal processing technologies using beams with a Gaussianintensity distribution, the alignment between the beam and the beamforming optical system may be adjusted by measuring the tilt in theresultant temperature profile. Such a measurement provides a directestimate of the amount the Gaussian beam is decentered on an aspheric ifthat is what is used to shape the image profile. Real time adjustmentsin the position of the beam on the aspheric may be made for renderingthe beam's intensity profile more uniform. In some instances, it may beadvantageous to measure the tilt in the temperature profile produced onthe wafer instead of measuring the beam decenter directly. Thefourth-power dependency of the emitted power as a function oftemperature makes this technique four times more sensitive thanmeasuring beam decenter or the beam profile on the substrate directly.In addition, direct measurement of substrate temperature mayautomatically compensate for a number of other effects such as a slowshift between the position determined by beam offset detectors and theactual center of the aspheric beam forming element.

Beam Breathing (Beam Size Changes)

As a related matter, beam-forming technologies used with the inventionmay also have to take into account or compensate for beam breathing.Beam breathing typically occurs when a continuous laser beam traverses asignificant distance though an atmosphere. Irregularities in thetemperature along the beam path through the atmosphere may create smallindex of refraction irregularities that cause the beam to dance aboutand to change shape. Some of the irregularities may also occur in thegas plasma contained with the laser cavity. The intensity profile of thebeam will likely change over time as well.

Detection of beam breathing is generally a straightforward matter,particularly for input beams with a generally Gaussian intensityprofile. For example, FIG. 4 illustrates the effect of a ±6% change inbeam size, which changes the uniformity by a like amount. As shown, anominally flat topped beam profile may be rendered concave or convex atthe top depending on whether the Gaussian input beam shrinks or expandswith respect to its nominal size. A measurement comparing the averageintensity or the temperature at two points equally spaced from the beamcenter with the intensity or temperature at the beam center leads to aconvenient measure of any change in the beam width.

To first order, beam breathing may be corrected for by incorporatinginto the beam path a bendable element that adds or subtractsoptical-power to the beam path thereby changing the beam size furtherdown the path where the beam shaping optics are located. Optionally, butnot necessarily preferably, the aspheric may be bent.

Apodization Modeling

Design, construction and implementation of optical modules may beginfrom parameters generated from ray-tracing modeling techniques known inthe art. FIG. 5 shows the geometries associated with an exemplary model.An aspheric reflective surface is shown having a profile that isdescribed by a function, f(x). A beam of radiation (input beam 112)strikes the aspheric surface at angle a relative to aspheric surfacenormal N_(A). In turn, the radiation (output beam 140) is reflectedtoward and strikes substrate surface, P, at angle θ relative tosubstrate surface normal N_(S), thereby forming an image. The intensityprofile of the image formed beam at the substrate surface I_(P)(p) canbe approximated by Eq. (1) as follows:I _(P)(p)=I _(x)(x)(Δx/Δp)  Eq. (1)where x represents the distance from the center of the aspheric surface,I_(x)(x) is the intensity profile of an image formed at the asphericsurface, p represents the distance from the center of the image formedon the substrate, and Δx and Δp are the relative sizes of the beams onthe aspheric and substrate surfaces, respectively, which are related asfollows:Δp=Δx(1−(2L/cos a)(d ² f(x)/dx ²))cos a/cos θ  Eq. (2)where L is the distance between the aspheric surface to the substratesurface and is assumed not to vary. The combination of Eqs. (1) and (2)yields Eq. (3), as follows:d ² f(x)/dx ²=(cos a−(I _(x)(x)/I _(P)(p))cos θ)/2L  Eq. (3)Experiment has shown that when the aspheric is replaced with a flat foldmirror, that the laser power has to be reduced considerably. Otherwisethe intensity in the beam center will be too high. This can be correctedif the center of the aspheric is made suffidently convex that the poweron the substrate is reduced by a factor of 1.4. Thus: When x=0:I _(x)(0)/cos a=1.4I _(P)(0)/cos θ  Eq. (4)Since it is desirable that I_(P) be constant, I_(P)(0)=I_(P)(p)=K. Thus:I _(P)(p)=(I _(x)(0)cos θ)/(1.4 cos a)=K  Eq. (5)d ² f(x)/dx ²=(cos a−1.4I _(x)(x)cos a/I _(x)(0))/2L  Eq. (6)andp=(x cos a−2L(df(x)/dx))/cos θ  Eq. (7)

By using appropriate values for L, a and θ, the equations set forthabove may be used to model intensity profiles at substrate surfaces. Forexample, a typical LTP system may be modeled assuming L=509.32, a=37.5°and E=75°.

In a simple model, an input beam having a Gaussian intensitydistribution is assumed. The representative plots of this model areshown in FIGS. 6-10. FIG. 6 provides a plot of the aspheric profile thattransforms a Gaussian profile into a flat profile at the substrate. Thisaspheric curve may be approximated by following equation:f(x)=−(1.5572×10⁻⁴)x ²+(1.5781×10)x ⁴−(5.2366×10⁻⁹)x ⁶+(1.2854×10⁻¹¹)x⁸−(1.6×10⁻¹⁴)x ¹⁰  Eq. (8)

Equation (8) is a five-term polynomial (x², x⁴, x⁶, x⁸, and x¹⁰)approximation of a surface of an ideal aspheric element suitable for usewith the invention when a beam having a Gaussian profile is employed. Inthis case, f(x) represents an approximation to the ideal surfaceprofile. The approximation is reasonably accurate for values of x below10. The profile is assumed to be symmetrical about x+0 so only half isshown in FIG. 6.

The aspheric profile starts out at its center with a negative curvatureto reduce the intensity on the wafer at the center of the beam by afactor of about 1.4 in addition to the reduction that occurs due to thelarge incidence angle on the substrate. An aspheric element with such aprofile may be used instead of a reduction optical relay (e.g., a 1.37×relay). As the distance from the center increases, the aspheric profilecurvature changes from negative to positive in order to concentrate thediminishing energy in the wings of the Gaussian intensity profile of theincident beam. As a result, the image formed on the wafer is rendereduniform in intensity.

FIG. 7 shows a plot of an idealized relationship between the rayposition on the substrate versus its position on the aspheric elementdescribed above. The ray position on the wafer becomes asymptotic withaspheric positions beyond 12 mm. Thus, ray tracing with an idealaspheric element predicts a perfect boxcar profile on the waferextending about 28 mm either side of the center. In practice the profileat the ends of the boxcar may be limited by diffraction, which is noteasily modeled using geometrical ray tracing techniques. Accordingly,the useful length of the image generated by the beam would be closer to±25 mm.

If the aspheric followed the five-term approximation in Eq. (8), whichdeparts slightly from the ideal curve beyond about 11 mm, then theresultant wafer intensity profile is as shown in FIG. 8. Thus theaspheric profile also assists in generating a sharp cutoff at the beamedge. The beam profile drops from 99% to 35% in going from 27.96 mm to28.519 mm on the wafer surface with this ray tracing model.

FIG. 9 provides the results from a ray trace model that examined theeffects of beam decentering relative to the aspheric on the intensityprofile of an image. When a beam has a nominal width of 12.6 mm measuredbetween the 50% points on the aspheric element, a 2% decenteringcorresponds to a lateral decentration of 0.2521 mm. This much decenterresults in a ±14% uniformity error at the edge of the image. Thus, thetolerance for decentration, assuming a 1% peak-to-peak maximum allowablenon-uniformity, is about ±0.009 micrometers. Also shown on FIG. 9 is thebeam profile as a result of a 5% decentration, which corresponds toroughly 5/2 the result of a 2% decentration. Thus, as discussed above,nonuniformity varies linearly with decentration, and measuring thedegree of tilt in the temperature profile leads to a direct estimate ofthe amount the Gaussian beam is decentered on the aspheric element.

Ray trace modeling techniques may also be used to provide a fairlyaccurate prediction of the effect of beam size changes on uniformity. Asshown in FIG. 10, a ±6% change in the beam width may result in a concaveor a convex beam profile that departs from ideal by ±13% at the edge ofthe beam (x=8 mm).

The ray trace equations discussed above, allow the aspheric surface f(x)to be determined from the intensity profile of the input beam and thedesired intensity profile of the output beam. As discussed above, thelaser beam may be assumed to follow a Gaussian profile. However it hasbeen observed that some high power CO₂ lasers generate beams havingintensity profiles that may be better approximated with aGaussian-Hermitian curve rather than a purely Gaussian curve. Inaddition, the intensity profile of such a beam may vary over time andwith the distance between the laser and the measurement point. Thus, ina more comprehensive analysis, input beams having non-Gaussian intensityprofiles would have to be accommodated.

FIG. 11 provides a normalized plot of three different beam intensityprofiles, I_(x1), I_(x2), and I_(x3) that represent the range of laserbeam profiles that are likely to be encountered on one commerciallyavailable laser. I_(x1), is the intensity profile of a perfectlyGaussian beam (Beam 1) and I_(x2) and I_(x3) are the intensity profilesof laser beams having different amounts of fourth orderGaussian-Hermitian contribution. These three intensity profiles havebeen used in the ray trace model to examine the effects of beam profilechanges on the beam image uniformity after reflection from an asphericelement. FIG. 12 provides a plot of the intensity profiles of the outputbeams produced as a result of reflection from an aspheric surfacedesigned to render intensity profile I_(x2) uniform. As expected, theintensity profile of the output beam generated from Beam 2, I_(P2), isrendered uniform. Notably the intensity the output beams generated fromBeam 1 and Beam 3, I_(P1) and I_(P3), respectively, deviate increasinglywith distance from the intensity at the image center. As shown, I_(P1)and I_(P3) deviate from I_(P2) by nearly 10% at about 23 mm from theimage center on the wafer. Thus small changes in the input beam profileslead to large changes in the beam profiles on the wafer.

It has been noted that beam profile changes produce a similar effect tobeam width changes and therefore the beam width may be adjusted tocompensate for changes in the beam profile. FIG. 13 illustrates theefficacy of this approach. What was a 9% uniformity issue has beenreduced to less than 1%, however in one case the intensity is headed upwhere irreparable damage would likely be done to the substrate. Thus, inorder to employ to employ width adjustment to compensate for profilechanges, it may be necessary to find some means of attenuating the edgesof the beam profile.

There are a number of ways in which this problem may be addressed. Forexample, radiation from the periphery of the beam may be prevented fromreaching the wafer by a baffle and a relay. This solution is far moredifficult than it appears, because of the image artifacts generated in afully coherent imaging system. Another way to address this problem is tomodify the aspheric profile f(x).

FIG. 14 shows a plot of the aspheric surface profile f(x) as describedabove and a plot of a modified aspheric surface profile f_(m)(x) thatmay be used to provide an improved roll-off in the edge profile of thewafer image. FIG. 15 shows the wafer image intensity profilescorresponding to the 3 beam profiles shown in FIG. 11 as a result ofreflection from the modified aspheric surface and with sizecompensation. For all three cases the intensity profiles of the outputbeams, i.e., I″_(P1), I″_(P2), and I″_(P3), are substantially uniform toabout 18 mm from center. In addition, none of the intensity profilesshow the sharp intensity increase described above that would lead toirreparable damage of some of the structures on the substrate.

Adjustable Apodization

The ray trace model results show that small changes in input beamprofile or in the aspheric profile can result in large changes inillumination uniformity. Thus, as a general rule, it may be moredesirable to employ adjustable optical elements with response times fastenough to make the best of whatever wavefront profile is encountered,rather than a fixed aspheric profile predicated on a perfectly knownbeam profile. For example, active optics may be used as well as opticalelements that include some more limited adjustment capability. Theactive or adjustable optical element may have a reflective surface that,for example, may be initially flat and adjustable to a cylindrical oraspheric profile. In the case where a surface starts out aspheric in itsneutral form, the adjusting mechanisms may be used as a means to perfectthe figure to account for minor deviations from system to system or fromtime to time. If the surface starts out flat, then the adjustments couldgo so far as to provide the aspherization necessary to transform aGaussian curve into a boxcar shape.

In any case, any of a number of adjusting techniques and mechanisms maybe used to alter how the beam forming means interacts with the beam. Forexample, an aspheric element may be bent slightly to lengthen or shortenthe image length. When appropriately coordinated with measurements ofthe annealing temperature, this adjustment could be used to compensatefor slow changes in the radiation source intensity or beam breathing. Insome instances, such adjustments may be automated and given a closedloop servo response rate approaching 100 Hz—a bandwidth sufficient tocorrect for the atmospheric transmission contribution to beam breathingthat may be encountered during the practice of the invention. As anotherexample of an adjusting technique suitable for use with the invention,changes in beam width due to transmission through a long atmosphericpath may be corrected by adding a little cylindrical power to anominally flat surface placed in the path of the beam before the beamreaches a beam shaping element, e.g., upstream of the beam shapingelement. Other beam forming means and techniques may be discovered uponroutine experimentation.

FIG. 16 shows in cross-sectional view an optical element in the form ofa power-adjusting mirror 122 that may be used to introduce variableamounts of cylindrical power into the beam to change the size of thebeam along one axis. The power-adjusting mirror 122 includes a frame 202holding a reflecting member 204 that includes a bendable optical surface206 (perpendicular to the plane of the paper on which FIG. 16 isprinted) and bending levers 208, a plurality of water-cooled stiffeners210, a spring 214, and an actuator 212. The reflecting member 204 may bemade of from any of a number of materials, including, for example,copper, aluminum or molybdenum. When actuator 214 is a piezoelectricdevice or device with similar performance capabilities, the actuator 214in combination with the spring may elastically deform the opticalsurface 206 in a manner that easily yields a closed loop servo responserate approaching 100 Hz. Such a response rate represents a bandwidthsufficient to correct for atmospheric generated perturbations. Beambreathing may also change the beam width in the scan direction, andthereby alter the maximum temperature produced on the substrate. Suchchanges can be automatically corrected by changing the laser power toachieve the desired maximum wafer temperature.

FIG. 17 shows another exemplary optical element in the form of adeformable mirror 124 having a flexible member 300 with a reflectivesurface 302 (and a means for adjusting the surface 304. As shown, themirror 300 has a nominally planar top reflective surface 302 that can beadjusted to take on a cylindrical aspheric shape; i.e. aspheric in oneplane only. The top surface 302 is rendered flexible in one plane by aseries of cuts 306. While any appropriate means may be used to adjustthe reflective surface 302, FIG. 17 shows an adjusting means 304 thatincludes a stiff base 308 having a plurality of adjustment screws 310and four cooling channels 312. The profile of the deformable mirrorsurface 302 may be adjusted to a desired shape 314 by using the threeadjustment screws 310. The aspheric mirror 124 is attached to the block308 at two points chosen according to the size of the incident beam sizeand the desired wafer focal plane intensity.

FIG. 18 shows yet another exemplary optical element having a surfacethat may be deformed into a desired surface profile to render a beamintensity profile more uniform. The element 124 includes a flexiblemirror 300 having a top optical, reflective, surface 302. The reflectivesurface 302, as shown in FIG. 18, is in its neutral, undistorted stateand is generally planar. The mirror element also includes a plurality oflongitudinal support elements 320 coupling the flexible reflectivemember 300 to a stiff, liquid-cooled base 308. Each support elementcontains a heating element 322. The stiff base block 308 may beoptionally liquid-cooled though cooling channels 312. The heatingelements 322 in each longitudinal support element are separatelycontrolled. In response to the heat from the heating elements 322, andthe heat load on the mirror surface 302 from the incident laser beam,each of the support elements 320 will reach a different equilibriumtemperature and will expand or contract to deform the optical surface302. By varying the heat generated in each longitudinal member it ispossible to generate an arbitrary profile on mirror surface 302.

The response rate of the adjustable deformable mirror illustrated inFIG. 18 depends on its scale. Assuming that the support elements 320 areabout 50 mm long and are made from copper with a thermal diffusivity of1.136 cm²/s, the time constant for approaching equilibrium after achange in one of the heating elements is estimated to be about 5.5seconds. The average temperature difference needed to generate a 5micrometer profile is given by 0.005/(50×16.5×10⁻⁶)=6° C. If the profileof the adjustable aspheric were adjusted to exactly equal the idealprofile at 9 equally spaced points, it seems likely that any residualerrors would be small, perhaps small enough to result in a beamintensity profile uniformity of about 1%.

As discussed above, there is a need in the art to provide a beam havingan intensity profile with a useful portion that is uniform to about 1%.However, very small errors in the aspheric profile will result insignificant uniformity errors. Thus, a plurality of means may be used toprovide gross and fine control over the beam forming process to producea useful beam intensity profile that exhibits high uniformity, in somecases exceeding 1% uniformity. In some instances, additional correctionmeans may include additional beam sources, e.g., a laser diode array, asdiscussed below.

Beam-Combining Technologies

The invention may also employ a plurality of beams in combination. Forexample, two or more independent lasers may be used to generate aplurality of beams that may be combined in such a manner to produce asingle contiguous image, e.g., a line image, on a wafer surface toeffect thermal processing. The beams may have the same or differentintensities and/or intensity profiles. In addition, the beams may or maynot have the same wavelengths. In some instances, laser diodes orlight-emitting diodes may be used.

When two or more beams, each having a Gaussian intensity profile, arecombined to form a single image, it is possible to significantlyincrease the energy utilization of the beams, often synergistically,without reshaping or otherwise modifying any of the beams. For example,energy utilization of an image created from a single beam having aGaussian intensity profile in the context of thermal semiconductorprocessing is typically about 11%. When two Gaussian laser beams arecombined to generate a single image, energy utilization of the beams mayincrease to about 36%. Additional beams may result in still higherenergy utilization. The useful portion of the combined image istypically longer and/or larger than any useful portions of images of thebeams by themselves. This model assumes that the beams may be addedincoherently. Accordingly, the beams may have to be derived fromdifferent lasers and some precautions may be needed to assure that theirfrequencies do not coincide in a manner that produces coherentinterference effects that compromise the image quality for thermalprocessing applications described herein.

For the shallow junction annealing application, it is desirable to forma line image that is uniform to 1% over an extended length. FIG. 19shows a two-beam system that may be used to produce such an image frombeams each having a Gaussian profile. As shown in FIG. 19, a first beam,as represented by rays 112A1, generally follows path A1, and a secondbeam, as represented by rays 112B1 generally follows path A2. Unlikepath A1, path A2 is bent by optical element 120. Assuming each beam hasa Gaussian intensity profile, then to avoid diffraction effects fromoptical element 120, the beams may be separated by at least six timesthe 1/e² radius (6 ω₀), rendered colinear, and brought to the substrateP at the right point in their paths as the beams merge together. Such abeam separation/combination geometry is not an absolute requirement, butgenerally it proves convenient to combine the beams near their waist tosatisfy the 6 ω₀ beam separation requirement.

The resulting intensity profile for either of the dual beams shown inFIG. 19 can be modeled as follows:I(r)=(2P/πω ²)exp(−2r ²/ω²)  Eq. (9)where P is the total power in the beam, ω is the beam width or radiusmeasured at some point along its extent at the 1/e² intensity point, andr is an arbitrary radius measured from the center of the beam.

In order for the two beams, when added incoherently, to produce anominally flat profile it is necessary that, at the midpoint between thetwo beams, the intensity of each is 0.6 of the maximum intensity of eachbeam. As a result, it is implied that in the case of two beams that areseparated by 6 ω₀ the condition for a uniform combination is given by:I(r)=0.6=exp(−2(3ω₀)²/ω²  Eq. (10)where ω₀ is the width of the beams at their waist. Solving Eq. (10) forω in terms of ω₀, it can be seen that:ω==5.936ω₀  Eq. (11)Thus, when the beams have overlapped and diverged to 5.936 times theirminimum size, they will provide a uniform result.

When the uniform part beam is incident on the wafer at 75°, it isdesired that the uniform part of the image stretch over a length ofabout 10 mm. This uniform area is equal to 0.8 ω. Thus:0.8 ω=10 mm (cos 75°)  Eq. (12)ω=3.235 mm.  Eq. (13)

The separation between the two beams is 6 ω₀, which translates to 3.27mm. The distance along the beam axis z where the overlap between theexpanding beams is optimum can be determined from the followingrelationships:ω(z)=ω₀(1+(z/z _(R))²)^(0.5)  Eq. (14)z _(R)=πω₀ ²/λ  Eq. (15)where z is the axial distance along the beam measured from the positionof the waist and λ is the wavelength of the laser beams. Using awavelength of 10.6 micrometers and Eqs. (14) and (15) leads to:z_(R)=88 mm  Eq. (16)andz=515 mm.  Eq. (17)

FIG. 20 shows a schematic representation of the intensity profile of thetwo beams near the optimum separation and the combined intensity. FIG.20 does not take into account the tilted focal plane which interceptsone beam before the other making it desirable to adjust the relativeintensities slightly. More than two beams may be used as well.

It is also possible to combine beams of widely different intensities andwavelengths to achieve a uniform annealing temperature profile. Forexample, a high-power laser beam that forms an image having asubstantially Gaussian intensity profile on a substrate may be used incombination with beams from a plurality of lower-power beams from LEDsor laser diodes. As discussed above, for any beam with a Gaussianintensity profile, effectively only the central portion of the beam withthe highest intensity can serve as a useful portion for thermalprocessing. The energy associated with the useful central portion isabout 11% of the total beam energy, and the energy associated with therest of the beam, i.e., about 89% of the total beam energy, iseffectively wasted. Here, the lower-power beams may serve to addadditional power to the lower intensity portions of the high-power beam.As a result, a greater portion of the substrate illuminated by the imagemay be heated to substantially the same peak temperature. In effect, theenergy from the periphery of the high-power laser beam may be combinedwith the energy from the lower-power beams instead of being effectivelywasted.

Interference Issues

Interference issues sometimes represent a significant hurdle in theimplementation of beam combining and beam shaping technologies. Ingeneral, coherent interference effects should either be avoided orminimized. For example, when two laser beams are combined to generate asingle image, interference will arise between the two beams when theirwavelengths are nearly equal. Such interference is particularlydetrimental if it occurs for a period of time comparable to the dwelltime of the laser beam over a point on the substrate, e.g., about onemillisecond. Thus, one way in which adverse interference effects may beaddressed is to “lock” the two wavelengths in such a way that a smalldifference always separates them. Nevertheless, the wavelengths of lasergenerated beams are generally determined by a number of differentparameters. For example, a laser's wavelength may be affected by, thestate of laser's gain medium (e.g., the laser gas pressure and/orpressure if the medium is a gas), the laser's construction (e.g., thegeometry of its cavity), and etc. Left alone, a laser beam's wavelengthwill tend to drift in time as the gain medium temperature, pressure andcavity dimensions vary.

Thus, when two laser beams are employed, it is unlikely that the twobeams will exhibit the exact same frequency and interfere with eachother at any single point in time. However, there is a finiteprobability that, at some point in time, the beams will exhibit the samefrequency, and when they do, they will generate interference patternfringes with nearly 100% modulation. As discussed above, suchinterference is particularly detrimental if it occurs for a period oftime comparable to the dwell time of the laser beam.

The detrimental effects of such interference phenomenon may be mitigatedif interference were to occur in a sufficiently short time periodcompared to the dwell time. For example, if the transitory period forsignificant interference effects is one tenth or less than the dwelltime, then the net result on the peak temperature, which depends on theintensity integrated over the whole dwell time period, is likely to benegligible. Accordingly, another way in which adverse interferenceeffects may be mitigated is to vary at least one laser wavelength, e.g.,in a sinusoidal, saw-tooth, or other oscillating manner, with respect tothe other at a sufficiently high rate such that any coincidence orcoincidences in wavelength will be short compared to the dwell time.

For example, when two or more lasers are used, the cavity length of theone or more lasers may be periodically varied so that the time spanwhere the wavelengths from separate lasers is likely to be close enoughto generate interference effects will be short compared to the dwelltime. Optionally, one of the laser beams in the group of lasers beingcombined may have a wavelength that is not varied.

As discussed above, the exact wavelength and therefore the frequency atwhich a laser operates depends on a number of parameters, one of whichis the length of the cavity. Within the gain curve, lasers can operateat frequencies (F) that are multiples of c/(2 L);F=nc/2 L=c/λ  Eq. (18)where c is the speed of light (3×10⁸ m/sec), L is the length of thecavity containing the gain medium, n is an integer (usually a very largeinteger), and λ is the wavelength.

For a one-meter long cavity, the laser frequency is an integer multiple(n) of c/2 L or 150 MHZ. These are called the longitudinal modes of thelaser. For example, for a laser operating at 10.6 micrometers, thecenter frequency is approximately 2.8×10¹³ Hz, or 2.8×10⁷ MHZ. If thelaser can operate at any integer multiple of c/(2 L), and L is equal to1 meter, then the multiple (n) is 188,679. The exact laser frequency forthis example is 2.830185×10¹³ Hz. The bandwidth of a longitudinal modeis determined by several factors, including the cavity geometry, the gaspressure and reflectivity of the mirrors. Typical bandwidths forlongitudinal modes in a gas laser can vary from a few to hundreds ofMHz.

As alluded to above, when the cavity length is varied, e.g., bydisplacing a mirror that determines the cavity length, the laserwavelength is also shifted. Using the above as an example, one mayassume a laser with a one-meter long cavity operating at 2.830185×10¹³Hz, and with a value of n=188,679. If the cavity length is increased byone micrometer, the laser frequency will shift (AF) to2.83018216981783×10¹³ Hz, and the laser frequency shift will beapproximately 28.3 MHz. The shift in laser frequency (ΔF) isapproximated by:ΔF=−F(ΔL/L)  Eq. (19)where F is laser frequency before a change of length of ΔL in the cavitylength L.

From this example, it should be evident that it is possible tosignificantly change the laser frequency with minute changes in thecavity length.

Thus, the invention may exploit the above effect to intentionallydestroy the interference effects from two (or more) lasers, each laserhaving a cavity length determined in part by a mirror position. Forexample, the rear mirror of one laser may be held steady and the rearmirrors of the other lasers are continuously displaced, each at adifferent frequency. The amplitude displacement of the mirror determinesthe amount of shift in the laser center wavelength for each laser andthe rate of this displacement will determine how quickly the centerwavelength moves. The minimum condition for avoiding interferenceeffects may be achieved by choosing a mirror displacement frequency thatdiffers from cavity to cavity by 1/D where D is the dwell time or thetime the laser beam takes to traverse a point on the substrate. That is,Δf≧1/D  Eq. (20)where Δf is the minimum shift in frequency from one laser mirror driverto the next, and D is the dwell time of the laser beam on a point on thesubstrate.

The minimum displacement amplitude of each mirror should be sufficientto produce a laser output frequency shift of about 9000(Δf) so that theproportion of time when the output frequencies of any two laserscoincide is a small proportion, ˜3%, of the dwell time. This yields thefollowing formula:ΔL≧9000Lλ/Dc  Eq. (21)

In an exemplary application using a laser with a one meter cavitylength, a 10.6 micrometer output wavelength, and a 1 millisecond dwelltime, the minimum driver oscillation amplitude works out to be 0.32micrometers, which corresponds to a 9 MHz oscillation in the laseroutput frequency.

There should be no harm in employing amplitudes much larger than thisand up to one quarter of the laser wavelength. For example, if twolasers are employed then one laser cavity could be left unmodulated, andthe other might be modulated at 1 KHz or higher frequency with anamplitude of 0.5 micrometer. This is a relatively easy task.

Thus, it is apparent that it may be a straightforward matter to avoidcoherence effects between multiple lasers within the dwell times ofcurrent interest for rapid thermal processing applications, e.g., on theorder of 1 ms, 10 μs, 100 ns, 10 ns, or less.

Other techniques for addressing interference issues are available aswell. For example, one may drive two or more cavity mirrors at the samefrequency 90° out of phase with each other so that when one laser isstable in frequency the other is rapidly changing. In addition, itshould be possible to destroy the coherence of a laser by is increasingthe number of transverse modes in the laser (i.e., increasing the valueM² of the laser). This is possible for certain classes of lasers, likesolid state lasers. See, e.g., U.S. Pat. No. 6,366,308 to Hawryluk etal. This approach, however, may not be practical using long gas laserswith few transverse modes.

Thus, variations of the present invention will be apparent to those ofordinary skill in the art. For example, the optical system of theinvention may employ a combination of any means known in the art formanipulating photonic beams. While the discussion above regardingapodization technologies generally focuses on various reflective beamforming means, refractive and/or diffractive beam forming means may beused as well. In addition, while the invention has been described ingreat detail in the context of one or two beam applications, theinvention is not limited to any particular number of photonic beams.

Upon routine experimentation, those skilled in the art may find that theinvention may be incorporated into existing equipment. For example, anadjustable aspheric mirror may be used to replace a fold mirror ofexisting equipment. In addition, auxiliary subsystems known in the artmay be used to stabilize the position and the width of the laser beamrelative to the optical system.

Furthermore, it should be emphasized that a temperature-feedback-basedcontroller, e.g., a servo-controller, may be advantageously used. Forexample, one may measure the temperature profile on a wafer with highresolution and using the measurements to correct the beam profile. Insome instances, such temperature measurement corrections may be carriedout in real-time. In addition, temperature measurements may be made togenerate a high-resolution map reflecting the thermal conditions underwhich the wafer as processed. For example, a map of the peak temperaturereached at each point on the wafer may be generated. Such mapping may becarried out with resolutions in about the millimeter to the micrometerrange. Such mapping may provide invaluable insight for furtherimprovements in the art of thermal processing, particularly in the areaof quality control and throughput.

It is to be understood that, while the invention has been described inconjunction with the preferred specific embodiments thereof, theforegoing description is intended to illustrate and not limit the scopeof the invention. Any aspects of the invention discussed herein may beincluded or excluded as appropriate. For example, beam combiningtechnologies and beam shaping technologies may be used by themselves orin combination. Other aspects, advantages, and modifications within thescope of the invention will be apparent to those skilled in the art towhich the invention pertains.

All patents and patent applications mentioned herein are herebyincorporated by reference in their entireties.

1. A method for processing a semiconductor substrate, the methodcomprising the steps of: (a) generating at least one photonic beamhaving a total beam energy and a nonuniform intensity profile; (b) usingthe at least one photonic beam to form an image on a surface of thesubstrate, the image exhibiting a substantially uniform intensityprofile over a useful portion thereof such that energy utilization bythe useful portion of the image is at least 15% of the total beamenergy; and (c) scanning the image across the surface of the substrateto heat at least a portion of the substrate at and/or near the surfaceto achieve a desired temperature within a predetermined dwell time, D.2. The method of claim 1, wherein the nonuniform intensity profile ofphotonic beam is substantially Gaussian in shape.
 3. The method of claim2, wherein the intensity profile of the photonic beam exhibits a profileconsistent with a curve generated from a set of Hermite-Gaussianpolynomials.
 4. The method of claim 1, wherein step (b) furthercomprises the step of: (d) manipulating the at least one photonic beamto render at least a portion of its intensity profile more uniform. 5.The method of claim 4, wherein step (b) is carried out such that theintensity profile of the image is rendered less Gaussian and morebox-car in shape that an intensity profile that would be producedwithout manipulation of the at least one photonic beam in step (d). 6.The method of claim 1, wherein step (a) generates a single photonic beamhaving a nonuniform intensity profile.
 7. The method of claim 1, whereinat least two photonic beams are combined to form a contiguous image. 8.The method of claim 7, wherein the contiguous image has a substantiallyuniform intensity profile over the useful portion that is longer thanprovided by any one of the at least two beams.
 9. The method of claim 1,wherein the energy utilization by the useful portion of the image is atleast about 25% of the total beam energy.
 10. The method of claim 1,wherein the energy utilization by the useful portion of the image is atleast about 35% of the total beam energy.
 11. The method of claim 1,wherein the desired temperature is an annealing temperature sufficientto electrically activate dopant atoms implanted into a semiconductormaterial.
 12. The method of claim 1, wherein the desired temperature isat least about 1300° C.
 13. The method of claim 1, wherein thepredetermined dwell time is no longer than about 1 millisecond.
 14. Themethod of claim 1, wherein the at least one beam is pulsed.
 15. Themethod of claim 1, wherein the at least one beam is continuous.
 16. Themethod of claim 1, wherein the substrate surface has a Brewster's anglefor the at least one beam and the at least one beam is incident to thesubstrate surface at or near the Brewster's angle.
 17. The method ofclaim 1, wherein the at least one beam has a power of at least 250 W.18. The method of claim 1, wherein the at least one beam has a power ofat least 1000 W.
 19. The method of claim 1, wherein the at least onebeam has a power of at least 3500 W.
 20. The method of claim 1, whereinscanning the substrate across the image provides an exposure energy doseof at least about 25 J/cm² when D is about 1 millisecond.
 21. The methodof claim 1, wherein the exposure energy dose is no more than about 60J/cm² when D is about 1 millisecond.
 22. The method of claim 1, whereinthe image has a peak intensity within the useful portion thereof and thesubstantially uniform intensity profile is entirely within a range ofabout 98% to 100% of the peak intensity.